its very unlikerly to be tiny, surfact rust is when you leave unprotected steel in the rain, a bubble had come from the other side of the panel its started off in the tray and spread all over inside and no has eaten right thru the skin to bubble up on the outside, the rs i just broke had no rust showing on the outside but was totally rotten inside as was a matter of time before it burst out

wind you roof back and peep in with a torch, get ready for a shock tho